Discussion

Session 2

Mr Z.J. Doron, Nordostschweizerische Kraftwerke AG

2 :a In trying to interpret Farmer's * boundary criterion, Adams and Faux discuss a 'safety index', Spc, which is used in a site-oriented manner. At NOK, a Swiss utility, a similar approach has been found very useful in a plantoriented analysis. The relative significance of each branch in a probability analysis of a given accident is determined by carrying it through till a frequency value and an iodine release value can be_assigned to it, with dimensions of 'year ' and 'curies' respectively. The product of these two gives a curies/year value, which we have called 'Mean annual severity' (MAS). Each branch.is thus described by a discrete MAS number(FlS) . A comparison of these numbers shows immediately which branches are significant contributors to the mean annual severity of an accident as a whole, and may therefore require further attention, and which branches are insignificant, and need be considered no more. MAS can be applied to a single branch of an analysis, to a complete accident or to the safety analysis of a whole reactor. The relationship between MAS and Farmer's boundary criteria can be seen in Fig 2:b taken from a paper by Doron and Albers2:D. The MAS valves correspond essentially to lines with a -1 slope on Farmer's logarithmic probability consequence diagram. However, whereas a boundary line may sanction a very large number of points very close to it, though on the low risk side, a discrete MAS number takes account not only of the value but also of the number of such points, or branches, by summing them up to a single number. This eliminates the temptation to take a point on the 'wrong' side of the boundary line and split it up into a number of points, each with a lower probability, which might then all lie on the 'right' side of the boundary.